Setting casing in wells



April 12, 1966 e. c. HOWARD SETTING CASING IN WELLS 2 Sheets-Sheet 1Filed April 15, 1963 ATTORNEY.

April 12, 1966 c HOWARD 3,245,471

SETTING CASING IN WELLS Filed April 15, 1963 2 Sheets-Sheet 2 I I l 1.34-" I I I 33* i i I l FIG-3 4a 36 K :drfi GEORGE 0. HOWARD.

INVENTOR.

Hilq 1 I %2z;

ATTORNEY United States Iateht ice 3,245,471 .SET'IING CASING IN WELLSGeorge C. Howard, Tulsa, Okla, assignor to Pan American PetroleumCorporation, Tulsa, kla., a corporation of Delaware Filed Apr. 15, 1963,Ser. No. 272,997 19 Claims. (Cl. 166-46) This invention relates tosetting casing in wells. More particularly it relates to forming at thetop of a well a seal between strings of easing.

It is customary in drilling or completing wells to run strings of steelcasing in the well to various depths. The principal purpose is to keepunwanted fluids from entering the well. A seal is ordinarily formedbetween the bottom of the casing and the well wall by placing cement inthe space between the casing and well wall. A seal is also required atthe top of the well between strings of easing. This may be provided bymeans of fianges and gaskets. A simpler system for forming a seal isdesirable. This is particularly true in marine locations where it isconsidered advisable to complete the well on the marine floor, leavingnothing extending above the surface of the water. In this case, diversare ordinarily required to manipulate the flanges and gaskets, place thebolts through the flanges and tighten the bolts. A system which could beoperated entirely from the surface of the water without divers wouldobviously be highly advantageous.

With these problems in mind, an object of this invention is to provide amethod for forming a seal between the tops of easing strings in a well.An additional object is to provide a method which will produce a sealbetween casing strings, the seal being capable of supporting the weightof the inner string. Still other objects will be apparent to thoseskilled in the art.

In general, I accomplish the objects of my invention by placing alongitudinally corrugated section of casing near the top of the innercasing string and expanding this corrugated section into a substantiallycylindrical shape inside the top of the outer casing string. A sealingmaterial such as copper or a glass fiber material saturated with asettable liquid resin surrounds the inner string opposite the corrugatedsection.

In the drawing,

FIGURE 1 is a cross-sectional view of the corrugated section of easingtogether with one form of apparatus for expanding the corrugated sectioninto substantially cylindrical form against the outer casing.

FIGURE 2 is another cross-sectional view of the corrugated casingsection with another form of apparatus for expanding the corrugatedsection.

FIGURE 3 is a cross-sectional view of the lower part of apparatus, thetop part of which is shown in FIG- URE 2.

Considering the drawing in more detail, in FIGURE 1, outer casing string11 and inner casing string 12 extend into a well. The upper portions ofthe strings may be in the well or above the well. Inner casing 12includes a corrugated section 13. Immediately above and below thecorrugated section the inner casing string is of a diameter which fitsclosely inside the outer casing. The corrugated section and the largeportions immediately above and below are manufactured as one joint orlength. The bottom portion of this length of casing is swaged down andthreaded to fit a collar 14 on the upper end of a regular size joint ofthe inner string of casing.

Above the corrugated section of casing an expander tool is provided.This includes a central mandrel 15 and expander cone 16 and expanderarms 17. The ends 18 of arms 17 rest on a tapered portion 19 of cone 16.The upper ends of arms 17 are held in a groove in mandrel 15 by a sleeve20. Above sleeve 20 a flexible upwardly facing cup 21 of rubber, forexample, is provided to form 5,245,471 Patented Apr. 12, 1966 a sealbetween mandrel 15 and inner casing 12. Attached to the top of mandrel15 is a length of tubing 22.

In operation the inner casing string is lowered into the well as usualuntil the bottom of the casing is only a few feet above the desiredlevel. The length of pipe including the corrugated section is thenattached to the top of the inner casing and the entire string is lowereduntil the corrugated inner casing section is inside the outer casingnear the top of the outer casing string. The bottom portion of the innerstring is then cemented in any of the usual ways. When the last of thecement has been displaced out the bottom of the casing by drilling fluidfor example, the expander tool is then placed in the position shown inFIGURE 1. A cementing head is then placed on the top of the inner casingto seal the top-of this string. The tubing 22 passes upwardly throughthe cementing head so that liquids may flow up the center of mandrel 15and tubing 22 as the expander tool is forced downwardly in the casing.In high-pressure wells at back-pressure valve may be placed in tubing 22to maintain the desired pressure within the inner casing string.

As soon as the expander tool is in place the cementing pumps are startedagain and. a liquid such as drilling fluid is forced into the innercasing 12 above the expander tool just as it is pumped in above the topcementing plug. The pressure acting against cup 21 forces the expandertool downwardly to expand the corrugated section into substantiallycylindrical shape. The function of the tool seems apparent from thedrawing. It is described in more detail and claimed in U.S. patentapplication S.N. 179,609 filed by R. P. Vincent on March 14, 1962, nowPatent 3,191,680. The expander head. is forced down a distance equal tothe length of the corrugated section of easing. This can be easilydetermined by watching the movement of the portion of tubing 22 whichextends above the cementing head. After the corrugated section has beenexpanded, the expander tool is lifted out of the well by means of tubing22.

In FIGURE 2 the outer and inner casing strings 11 and 12 are much thesame as in FIGURE 1. Again a corrugated section 13 is provided in theinner string between enlarged portions immediately above and below. Theenlarged portion immediately below the corrugated section is long enoughto-contain the expander tool. In FIGURE 2, however, the bottom of thelength of casing which includes section 13 is not swaged down. Instead,it is internally threaded to fit a swaged nipple 30 which connects tocollar 14 on the regular size casing.

The expander tool shown in FIGURE 2 is actuated by a hydraulic pistonand cylinder system. The tool includes a polished rod 31 extendingthrough expander cone 32 and collet head 33. Collet head 33 includesspring arms 34 with upper ends 35'extcnding up into a retaining groovein the bottom of cone 32. On the lower end of polished rod 31 is apiston and cylinder assembly 36. The top of polished rod 31 is attachedto tubing 22.

In operation of the apparatus shown in FIGURE 2, an inner casing stringis run and cemented as usual without the corrugated section of easing orthe swaged nipple. In their place an ordinary length or joint of casingis used. After the cement has ,set, the top joint of the inner string isbacked off and replaced by the length containing the corrugated sectionand the swaged nipple. It will be noted that in this in this embodimentthe expander tool is placed in the enlarged portion of casing below thecorrugated section before the length of easing, including both theenlarged and corrugated sections, is attached to the top 'of the regularcasing string. When the attachment has been made, the inner casingstring is again placed in tension, any previous tension having beenreleased to remove the top standard joint of casing and replace it withthe special joint. Liquid is then pumped through tubing I 22 andpolished rod 31 to the piston and cylinder assemly. The rising cylinderhead then forces the expander cone and spring arms through thecorrugated section of casing after which. the expander tool is removedfrom the top of the casing. Apparatus of the type shown in FIG- URE 2 isdescribed in detail and is claimed in U.S. patent application S.N.384,017, filed by R. P. Vincent on June 25, 1964, new Patent 3,203,483.U.S. application S.N. 384,017 is a division of US. patent applicationS.N. 216,- 949 filed on July 10, 1961, now Patent 3,179,168.

Certain limits must be observed with regard to the corrugated section ofthe inner casing. For example, the external cross-sectional perimeter ofthe corrugated section must be greater than the internal circumferenceof the outer casing. This insures that after the corrugated section isreformed into substantially cylindrical shape, it will be in maximumcompressive stress in a circumferential direction. The purpose is toform and hold a strong seal between the two strings of casing. The outercasing must, of course, be in sufficient tensile stress to hold theinner casing in maximum compressive stress. It will be obvious then thatif both strings are of the same material or alloy, which is usually thecase, the inner string must be thinner than the outer one, at least inthe interval of the corrugated section.

Actually, the outer casing should be considerably thicker than the innerone at the level of the corrugated section. This is because after theexpander arms on the expander tool have reformed the liner intosubstantially cylindrical shape, these arms continue to exert radialforce. Thus, the outer casing must withstand not only the stress imposedby the inner casing but also the stress imposed by the arms of theexpander tool. Ordinarily the stress resulting from-the expander arms ismuch less than that applied by the liner. In order to have some marginof safety, however, it is generally advisable to use a steel corrugatedsection which is not more than about /2 as thick as the steel outercasing in which it is set.

As a practical matter, the thickness of the internal casing willordinarily be at least slightly smaller than the thickness of the outercasing. It may be advisable, however, to form the corrugated sectionfrom pipe having an even smaller wall thickness than that of the innercasing string. It is also possible to provide an extra thick section ofpipe at the top of the outer casing string opposite the interval inwhich the corrugated section is to be expanded. In this way it ispossible to make the corrugated section of the same wall thickness asthe remainder of the inner casing string.

It is also possible to make the corrugated section of a metal somewhatweaker than the metal of the outer casing string, or to make the outercasing string, opposite the corrugated section, of an extra strongmetal. Either expedient permits a wider choice of wall thicknesses foreither the corrugated section or the outer casing string opposite thecorrugated section.

If the casing strings are made of different types of steel, they may nothave the same strengths. The more general limitation on thickness can bestated, therefore, that the maximum compressive stress of the materialof which the corrugated section is made times the wall thickness of thissection must be less than the maximum tensile strength of the materialof which the outer casing string is made times the Wall thickness of theouter casing in the interval in which the corrugated section is set.Generally, a corrugated section thicknessof only about /2 the indicatedvalue should be used to allow for a margin of safety.

The amount by which the corrugated section wall must be compressed toinsure that the expanded section reaches maximum compressive stress isusually less than about A of 1 percent. When a layer of glass fibers andplastic, or of a metal such as copper, aluminum or the like, is providedbetween the corrugated section and the outer casing, and if the externalperimeter of the corrugated section is any greater than the internalcircumference of the outer casing, this requirement is satisfied. 'Ifthe corrugated section external perimeter is more than about 10 percentgreater than the internal circumference of the outer casing, the workrequired to reform the liner into cylindrical shape may be great, so alarge force may be required to move the expander head through thecorrugated section at a desirable rate. Therefore, the corru gated.section external perimeter ordinarily should not exceed the outer casinginternal circumference by more than about 10 percent.

The length of the corrugated section may be as short as a foot or two ifa glass fiber mat filled with plastic is used between the casing string,and if some means other than the seal has been provided for holding theinner string in tension. If the seal is also to support at least aportion of the weight of the inner string of easing, hovever, a lengthof at least about 4 or 5 feet is recommended. A length of about 10 feetis ordinarily preferred for most circumstances when using the glassfiber mat and plastic seal. Greater lengths may, of course, be usedifdesired. Many glass fiber mats and satisfactory liquid resins capable ofbeing set to a hardened state are available. In general, woven glasscloths and epoxy resins are preferred.

If a metallic sealing sleeve is'used between the casing strings, thelength of sealing sleeve and corrugated section should be considerablygreater than when a glass'mat and plastic is used. This is because themore deformable metals such as copper, aluminum, and lead, or evenstrong alloys of these metals, tend to creep under pressure and thusrelease the compressive stress in the inner casing string, at least nearthe end of the sealing sleeve. Thus, the length of the corrugatedsection and corresponding sealing metallic sleeve behind it, shouldordinarily be at least about 5 to 10 feet and preferably around 15 to 20feet in length or even longer.

When the corrugated section is reformed into substantially cylindricalform, the metal is in a condition of plastic flow. Under thesecircumstances the metal of the corrugated section may flow enough tomake a sufiiciently effective seal against the inner surface of theouter casing string. This is particularly true if the well is alow-pressure well and if the outer string of casing is new and has asmooth inner surface. Ordinarily, however, a sealing material should beused between the corrugated section and the outer string of easing. Ifthe sealing material is a deformable metal, it will be apparent thatthis metal should be more deformable than the steel of which thecorrugated section is made. Therefore, when reference is made to adeformable metal it will be understood that a metal or alloy moredeformable than steel is intended.

The length of inner casing containing the corrugated section must bespecially made. The corrugated section is made of pipe slightly largerin external circumference than the internal circumference of the outercasing. Immediately above and below the corrugated section are portionswhich fit as closely as possible inside the outer casing and stillretain some freedom of vertical movement. On the bottom of the speciallength, some means, such as those previously described, must be providedto connect the special casing length to the regular internal casingstring in the well. On the top of the special length, means may beprovided for making the usual connections to support well tubing or thelike. It i also possible, however, to cut the inner casing above thereformed corrugated section so connections can be made to the upper endof the outer casing string.

It is important that the inner string of casing be placed in tensionbefore the seal at the upper end is made. This is to prevent buckling ofcasing when hot well fluids flow up through the tubing and the casing isthus heated. Ordinarily there is a relatively simple solution to theproblem. The inner string is generally hung on elevators and the weightof the casing provides the necessary tension. It should be noted,however, that considerable force is required to move the expander headthrough the corrugated section. Therefore, if the expander is to bemoved downwardly, it may be necessary to provide sufficient support notonly for the weight if the casing but also oppose the downward force ofthe expander tool. In the apparatus in FIGURE l'the downward force iscompensated for by the upward force on the cementing head. If a pistonand cylinder arrangement is used to force the expander down, however, itmay be necessary to oppose this force. If the expander tool is movedupwardly, it is generally advisable to release at least a small amountof the lift on the upper end of the inner casing to avoid any possibleexcessive stretchingof the inner string.

The amount of tension in the inner string of casing ordinarily is notparticularly critical. Therefore, it is possible simply to hang theliner string from a flange or other supporting structure resting on thetop of the outer casing string or similar fixed support. This isparticularly important in marine completions where the work is beingcarried out from a floating platform. In this case it is diflicult,although not impossible, to hold a sufiiciently constant tension on theinner string of casing while it is being .cerneted and the top seal isbeing formed. It is much better in such cases to support the weight ofthe innerstring on a flange resting on the outer string of like, i

r The expander :arms of the tools shown in FIGURES 1 and 2 aredifferent. In FIGURE 1 the arms are pivoted at one end and are forcedoutwardly by inclined surfaces at the other end. In FIGURE 2 the armsare spring arms. It is possible, of course, to use the pivoted armdesign in the apparatus of FIGURE 2 and the spring arm design in theapparatus of FIGURE 1. It is also possible to invert the hydraulicpiston and cylinder ofFIG- URE 2 and use it to drive the expander coneand arms of either FIGURE 1 or FIGURE 2 downwardly through thecorrugated section. Rather than using hydraulic means to force theexpander head through the corrugated section, it is also possible to usethe tubing to pull or push the expander head. Still other possibilitieswill be apparent to those skilled in theart. Some modifications will, ofcourse, sometimes be required. For example, a spring between thetrailing edge of cone l6 and the mandrel shoulder behind this end inFIGURE 1 should be used to force the cone away from the expander arms ifthis form j of expander tool is to be pulled upwardly through thecorrugated section.

Each system has its advantages and disadvantages, F or example, thearrangements in which the expander tool is pulled upwardly through thecorrugated section require the top joint of the inner casing to beremoved and replaced with the special length containing the expandertool below the corrugated section. This is perfectly satifactory for useon dry land and for many marine locations. It may not be easily applied,however, where a floating work platform is used. Such a system is easilyadapted to use in old wells where at least the top portion of the innercasing string is removed and replaced with a.-

I new section. 1 In this casing no cementing step is required, so thespecial joint of casing containing the expander too] can be run inimmediately.

Any of several well-known methods can be used to remove the top joint ortwo of the inner casing string and replace this portion with the speciallength. For example, a le-fthand thread may be provided in the upperpart of the collar to which the special length is to be attached. It mayalso be possible to cause the casing to jump loose from the collar byapplying tension to the casing string and using a small explosive chargeopposite the collar. It is also possible to out the inner casing stringat the desired level and use a casing bowl repair tool to connect thespecial length to the cut end of the inner string.

Many other schemes will be apparent to those skilled in the art.

The spring arms have the disadvantage that after they have expanded thecorrugated section, they may continue to press on the uncorrugatedportion of the inner string as the expander tool is withdrawn from thewell. The pivot arms have the disadvantage that they are released assoon as the solid cone emerges from the end of the corrugated se-ction.A short length of the corrugated section is thus left in a partiallyexpanded condition.

In this connection, when reference is made to expanding or reforming thecorrugated section into substantially cylindrical shape or form, it willbe understood that the entire length of the corrugated section need notbe completely expanded, but only enough to provide the desired seal andsupport. The disadvantage with neither apparatus is serious for mypurposes. The disadvantages can, however, be avoided by using a somewhatmore complex expander tool.

The disadvantage with the spring arms can also be avoided by using anextra thick section of the outer casing string opposite the corrugatedsection'of the inner string. This will mean that the internal diameterof the outer spring opposite the corrugated section will be smaller thanthe diameter immediately above this section. It will be possible,there-fore, to use a section of theinternal string immediately above thecorrugated section, larger in diameter than the diameter of the reformedcorrugated section. This will at least reduce, and may avoid, thepressure of the spring arms after the expander tool has been pulled upthrough the corrugated section.

In applying my method to forming .top seals between casing strings inwells in marine location-s, the specific technique to be used willdepend to some extent on the type of working platform available and thetype of Well head desired. If the platform is a permanent one and thewell head is to be on the platform, then any of the suggested techniquescan be used. If a temporary platform is used, the well head willprobably be below the surface of the water. In this case, my sealingmethod has the advantage of avoiding the use of divers to makeunderwater connections, but again any of thetechniques suggested abovecan be used since a firm platform is available from which the positionof, and tension in, the inner casing string can be controlled.

If a floating work platform such as a ship is used, a submarine wellhead will be used and my method again will have the advantage ofavoiding the use of divers to complete the seal between casing strings.In this case, however, the rising and falling of the floating platformwith Waves and tides make it difficult to control the position of, andtension in, the inner casing string. Therefore, it is best in this caseto hang the inner string on a supporting structure such as a flangeresting on the top of the outer casing string or other support on themarine floor. An expander tool having a sealing cup as shown in FIG- URE1 can then be pumped down the internal casing string, which includes aslip joint in the interval between the floating platform and the marinefloor, to expand the corrugated section of casing and form the desiredseal.

Many additional embodiments and variations will occur to those skilledin the art. Those presented above are given by way of example only.

I do not wish to be limited to these specific examples, but only by theterms of tudinally corrugated malleable metallic section near the top ofsaid inner string at a location within said outer string, the wallthickness of said outer string, opposite said corrugated section, timesthe maximum tensile v strength of the material of which said outerstring, opposite said corrugated section, is made being greater than thewall thickness of said corrugated section times the maximum compressivestrength of the material of which said corrugated section is made, theouter cross-sectional perimeter of said corrugated section being greaterthan the internal circumference of said outer string opposite saidcorrugated section, placing at least the top portion of said innerstring of casing in tension, and while said inner string is in tension,expanding said corrugated section into a substantially cylindrical shapeinside said outer string of casing.

2. The method of claim 1 in which the external crosssectional perimeterof said corrugated section is no more than about percent greater thanthe internal circumference of said said outer string of casing oppositesaid corrugated section.

3. The method of claim 1 in which said corrugated section is expanded byplacing in said inner string of easing above said corrugated section anexpander tool including an element to form a seal against the insidesurface of said inner string of casing, closing the top of said innerstring of casing and applying sufiioient pressure inside said innercasing string above said expander tool to cause said expander tool tomove through said corrugated section.

4. The method of claim 1 in which said corrugated section is surroundedwith a sealing material before said corrugated section is expanded.

5. The method of claim 4 in which said sealing material is a glass fiberm-at saturated with a liquid resin capable of being set to a hardenedstate.

6. The method of claim 4 in which said sealing material is a deformablemetal.

7. A method for forming, near the top of a well, a seal between an innerstring of casing and an outer string of casing in said well comprisingremoving at least the top portion of said inner string, replacing theremoved portion with a new portion including a longitudinally corrugatedmalleable metallic section within said outer section, the wall thicknessof said outer string, opposite said corrugated section, times themaximum tensile strength of the material of which said outer string,opposite said corrugated sect-ion, is made being greater than the wallthickness of said corrugated section times the maximum compressivestrength of the material of which said corrugated section is made, theouter cross-sectional perimeter of said corrugated section being greaterthan the internal circumference of said outer string opposite saidcorrugated section, placing at least the top portion of said innerstring of casing in tension, and while said inner string is in tension,expanding said corrugated section into a substantially cylindrical shapeinside said outer string of casing.

3. The method of claim 7 in which said corrugated section is surroundedwith a sealing material before said corrugated section is expanded.

9. A method for easing a well which already contains a string of easingcomprising lowering an inner string of casing into said well, throughthe outer string of casing, placing cement around the bottom of saidinner string to form a seal between the bottom of said inner string andthe well wall, removing the top portion of said inner string from saidwell, replacing the removed portion with a new portion including alongitudinally corrugated malleable metallic section within said outerstring, the wall thickness of said outer string, opposite saidcorrugated section, times the maximum tensile strength of the materialof which said outer string, opposite said corrugated section, is madebeing greater than the wall thickness of said corrugated section timesthe maximum compressive strength of the material of which saidcorrugated section is made, the outer cross-sectional perimeter of saidcorrugated section being greater than the internal circumference of saidouter string opposite said corrugated section, placing at least the topportion of said inner string of casing in tension, and while said innerstring is in tension, expanding said corrugated section into asubstantially cylindrical shape inside said outer string of casing.

10. The method of claim 9 in which said corrugated section is surroundedwith a sealing material before said corrugated section is expanded.

H. A method for easing a well which already contains a string of casingcomprising lowering an inner string of easing into said Well, throughthe outer string of easing, said inner string including near the top alongitudinally corrugated malleable metallic section, the wall thicknessof said outer string, opposite said corrugated section, times themaximum tensile strength of the material of which said outer string,opposite said corrugated section, is made being greater than the wallthickness of said corrugated section times the maximum compressivestrength of the material of which said corrugated section is made, theouter cross-sectional perimeter of said corrugated section being greaterthan the internal circumference of said outer string opposite saidcorrugated section, continuing lowering said inner string until saidcorrugated section is within said outer casing, placing cement aroundthe bottom of said inner string to form a seal between the bottom ofsaid inner string and the well wall, placing at least the top portion ofsaid inner string in tension, and while said inner string is in tension,forcing an expander tool downwardly through said corrugated section toreform said corrugated section into substantially cylindrical shapeinside said outer string of casing.

12. The method of claim 11 in which said corrugated section is expandedby placing in said inner string of casing above said corrugated sectionan expander tool including an element to form a seal against the insidesurface of said inner string of casing, closing the top of said innerstring of casing and applying sufficient pressure inside said innercasing string above said expander tool to cause said expander tool tomove through said corrugated section.

13. The method of claim 11 in which said corrugated section issurrounded with a sealing material before said corrugated section isexpanded.

14. A method for casing a well from a floating platform in a marinelocation, where said well already contains a casing string, comprisinglowering an inner string of easing into said well, through the outerstring of casing, said inner string including near the top alongitudinally corrugated malleable metallic section, and said innerstring also including a means for hanging said inner string on the topof said outer string, the wall thickness of said outer string, oppositesaid corrugated section, times the maximum tensile strength of thematerial of which said outer string, opposite said corrugated section,is made being reater than the wall thickness of said corrugated sectiontimes the maximum compressive strength of the material of which saidcorrugated section is made, the outer cross-sectional perimeter of saidcorrugated section being greater than the internal circumference of saidouter string opposite said corrugated section, continuing lowering ofsaid inner string until said corrugated section is within said outerstring and said means for hanging said inner string comes to rest on thetop of said outer string and supports said inner string, placing cementaround the bottom of said inner string to form a seal between the bottomof said inner string and the well wall, and forcing an expander tooldownwardly through said corrugated section to reform said corrugatedsection into substantially cylindrical shape inside said outer string ofcasing.

15. The method of claim 14 in which the external cross-sectionalperimeter of said corrugated section is no more than about 10 percentgreater than the internal circumference of said outer string of casingopposite said corrugated section.

16. The method of claim 14 in which said corrugated section is expandedby placing in said inner string of casing above said corrugated sectionan expander tool including an' element to form a seal against the insidesurface of said inner string of casing, closing the top of said innerstring of casing and applying suflicient pressure inside said innercasing string above said expander tool to cause said expander tool tomove through said corrugated section.

17. The method of claim 14 in which said corrugated section issurrounded with a sealing material before said corrugated section isexpanded.

18. The method of claim 17 in which said sealing material is a glassfi-ber mat saturated with a liquid resin capable of being set to ahardened state.

19. The method of claim 17 in which said sealing material is adeformable metal.

References Cited by the Examiner UNITED STATES PATENTS Leonard 166206 XSimmons 16 6-207 X Price 16'649 Minor et al. 16647 Bannister 16 5-2 07 XKnapp et a1 166-6 6.5 Jennings 16646 CHARLES OCONNELL, Primary Examiner.

C. D. JOHNSON, Assistant Examiner.

1. IN A METHOD FOR CASING A WELL IN WHICH METHOD AN INNER STRING OFCASING IS RUN INSIDE AN OUTER STRING OF CASING AND A SEAL IS FORMEDBETWEEN THE STRINGS AT THE TOP OF THE WELL, THE IMPROVEMENT COMPRISINGINCLUDING A LONGITUDINALLY CORRUGATED MALLEABLE METALLIC SECTION NEARTHE TOP OF SAID INNER STRING AT A LOCATION WITHIN SAID OUTER STRING, THEWALL THICKNESS OF SAID OUTER STRING, OPPOSITE SAID CORRUGATED SECTION,TIMES THE MAXIMUM TENSILE STRENGTH OF THE MATERIAL OF WHICH SAID OUTERSTRING, OPPOSITE SAID CORRUGATED SECTION, IS MADE BEING GREATER THAN THEWALL THICKNESS OF SAID CORRUGATED SECTION TIMES THE MAXIMUM COMPRESSIVESTRENGTH OF THE MATERIAL OF WHICH SAID CORRUGATED SECTION IS MADE, THEOUTER CROSS-SECTIONAL PERIMETER OF SAID CORRUGATED SECTION BEING GREATERTHAN THE INTERNAL CIRCUMFERENCE OF SAID OUTER STRING OPPOSITE SAIDCORRUGATED SECTION, PLACING AT LEAST THE TOP PORTION OF SAID INNERSTRING OF CASING IN TENSION, AND WHILE SAID